This invention relates to apparatus for delivering an electrical shock in the treatment of cardiac disorders. Cardiac arrythmias may be treated by the use of various drugs such as quinidine, procainamide, digitalis and the like. Additionally, electrical depolarizing impulses of varying voltage and current are utilized in reverting arrythmias.
Apparatus for delivering these electrical depolarizing impulses falls generally into two categories being AC defibrillators and DC defibrillators. During the period since the early 1960's, utilization of DC defibrillators has gained over the utilization of AC defibrillators for a variety of medical reasons. Several of these are reported in the article from the American Journal of Cardiology August 1962, Volume X, No. 2, pages 223-233 entitled "Comparison of Alternating Current with Direct Current Electroshock Across the Closed Chest", Bernard Lown et al.
The DC defibrillator utilizing electroshock is presently available from a variety of sources and can include in addition to the basic circuitry for generating, storing and delivering the shock many additional features. Among these additional features may be apparatus for monitoring the EKG wave of the patient to whom the electroshock is to be delivered. Additionally, it has been determined that since the effect of the electrical shock in restoring or stabilizing heart action is dependent upon the portion of the cardiac cycle in which the shock is delivered, synchronizing apparatus is often included to avoid the shock being delivered during that portion of a cardiac cycle in which adverse effects of the shock are anticipated. Apparatus having this capability is sold in one form under the trademark "Cardioverter" and is available from American Optical Corporation, Medical Division, Bedford, Massachusetts.
In addition to determining that the effect of the electrical discharge upon restoration of the cardiac cycle is dependent upon the time of the cycle in which the shock is delivered, it has also been determined that the waveform of the discharge has a substantial impact upon the beneficial effect of the electrical shock. In DC defibrillators, the electrical charge for delivery to the patient is stored in a capacitor which is charged either from an AC source or a DC source such as a battery. Since the customary waveform of a discharge from the capacitor is not optimum for cardiac resuscitation or synchronized defibrillation, wave shaping elements are included in the discharge circuit so that the appropriate wave shape may be delivered to the resuscitating paddles which are applied to the patient's chest area. Conventional DC defibrillators often include a charged storing capacitor of a size of approximately 16 microfarads and include serially in the discharge circuit an inductor having a value of approximately 100 millihenries. In the synchronized defibrillators available from American Optical Corporation, the waveform provided is that developed by Dr. Bernard Lown. It is characterized by the rise time of more than 500 microseconds to a peak value of less than 3000 volts. The duration of the waveform is approximately 5 milliseconds and is further characterized by a momentary undershoot of the opposite polarity on the trailing edge of the waveform. It is felt by Dr. Lown that this general form aids in the restoration of the heart's electrolyte balance. Conventional defibrillators either synchronized or otherwise deliver either similar waveforms or those suggested by others such as Edmark which appreciably vary from the overall characteristics above described.
The use of DC defibrillators has seen wide acceptance in recent years in portable apparatus which is carried by emergency rescue teams since the electrical shock delivered by the DC defibrillator can be provided to the charging capacitor by battery. The instrument is fully portable and may thus be carried in trucks, ambulances or the like where the instrument may be plugged into vehicular power (6 to 12 volt DC) or may be operated from the self-contained battery.
Attendant with the wide acceptance of portable units by emergency medical service units, increased emphasis has been placed on manufacturers to produce lightweight, extremely-compact units. Accordingly, a portion of this compactness has been achieved from the use of newly developed capacitor storage devices which include new dielectric materials permitting closer placement of the capacitor plates.
Additionally, with the wide application of this instrumentation in field units, the practice has developed of testing the functionality of the unit by shorting the paddles together and discharging the unit therethrough.
We have determined that while this practice has continued for several years now, prior to our discovery of its impact on the deterioration of life of DC defibrillators this accelerated deterioration in general was not resolved. We have determined that during the course of discharge through the shorted paddles, a voltage ringing occurs. This voltage ringing manifests itself in current reversals through the discharge circuit including the wave-shaping inductor and apparently has a substantial adverse effect on the charge storage capacitor. The voltage reversals currently reach high potential, e.g., approximately double that normally stored on the capacitor. We have determined that this repeated voltage reversal manifests itself in the breakdown of the dielectric material in the capacitor, rendering the defibrillator inoperative.
The present invention overcomes the problems discovered which are inherent in the discharge of the DC defibrillator directly through the paddles. The improvement is accomplished without material impact upon the performance of the synchronized defibrillator during the therapeutic function, namely, the amount and waveform of the electrical shock is not affected by the solution to the problem as perceived by us.